The present invention relates generally to exposure apparatuses, and more particularly to an exposure apparatus used to expose an object such as single crystal plates for semiconductor wafers, glass plates for liquid crystal displays (LCDs), and the like. The present invention is suitably applicable, for example, to an exposure apparatus for exposing single crystal plates for semiconductor wafers in a step-and-scan projection manner in a photolithography process.
The step-and-scan manner, as used herein, is one mode of projection exposure method which exposes a mask pattern onto a wafer by continuously scanning the wafer relative to the mask or a reticle (which are used interchangeably in the present application), and by moving, after a shot of exposure, the wafer stepwise to the next exposure area to be shot.
Along with the recent demands on smaller and lower profile electronic devices, minute semiconductor devices to be mounted onto these electronic devices have been increasingly required. For example, a design rule for a mask pattern requires that an image with a size of a line and space (LandS) of 0.1 xcexcm or less be extensively formed and, presumably, it will further move to a formation of circuit patterns of 80 nm or less in the future. LandS denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus, which is a typical exposure apparatus for fabricating semiconductor devices, generally includes an illumination apparatus that includes a light source, such as a laser, and an illumination optical system for illuminating a mask, and a projection optical system that is located between the mask and an object to be exposed. In order to provide a uniformly illuminated area, the illumination optical system introduces a beam from a laser to an optical integrator, such as a fly-eye integrator, to use multiple light sources created at and around the optical integrator""s light exit plane as a secondary light source to illuminate a mask plane via a condenser lens. More specifically, referring to FIG. 9, an optical integrator 130A uniformly illuminates, via a condenser lens 140A, a masking blade 160A for restricting an exposure area to be scanned. The beam that has passed through the masking blade 160A illuminates the illuminated plane of the mask through an image-forming lens (not shown). Here, FIG. 9 is an enlarged side view showing part of a conventional illumination apparatus 100A. This structure arranges an exposure plane on the wafer, a pattern plane on the mask, and a plane of the masking blade 160A in a conjugate relationship.
In such an exposure apparatus, the image quality of a pattern to be transferred onto the wafer and the like depends deeply upon illumination performance, e.g., illumination distributions on the mask and wafer planes. Therefore, precise light amount control (i.e., illuminance control) is required to provide high quality semiconductor wafers, LCDs, thin-film magnetic heads, etc. Such light amount control uses a beam splitting member (e.g., a beam splitter half mirror) 150A and the like to split the condensed beam subsequent to the condenser lens 140A, and a sensor 182A to receive it for feedback control over an amount of the light from the light source (or a laser output) so that the illuminance of an illuminated area may fluctuate within a permissible range. In such an illumination apparatus, the sensor 182A detects the light amount of the exposure beam while arranging its light receiving surface at a position corresponding to mask""s pattern plane.
However, there has recently been found a problem that an illumination apparatus that uses a half mirror distorts a beam intensity distribution, and causes distortion in an effective light source. As a result, an exposure apparatus using such an illumination apparatus deteriorates image quality on a wafer plane, deteriorating quality of devices to be provided.
As shown in FIG. 9, the half mirror 150A is arranged while somewhat inclined to a plane perpendicular to an optical axis OA so that a light-receiving element 182 for receiving a reflected beam may not obstruct exposure light. Therefore, it is understood that beams L1 and L2 that provide the maximum aperture (which beams L1 and L2 are referred to as an upper line L1 and a lower line L2 of the maximum aperture ray hereinafter in this application) make the largest difference in angle of incidence upon the half mirror among condensed beams incident upon the plane of incidence of the half mirror 150A.
Here, if it is assumed that the half mirror 150A is made of quartz (having an refractive index of 1.56 to a wavelength of 193 nm) and keeps its light receiving surface uncoated, the angular dependency of its spectral reflectance is as shown in FIG. 10. Here, FIG. 10 is a graph showing the spectral reflectance at the plane of incidence of the half mirror 150A. Thus, if the half mirror 150A""s plane of incidence is not set to be optimally inclined against the plane perpendicular to the optical axis OA by taking into consideration numerical apertures (NAs) of the preceding and following optical systems, the beam reflected from the half mirror 150A would include a distorted intensity distribution. This would also cause a distorted intensity distribution in a transmitting beam through the half mirror 150A, providing a distortion in an effective light source, and finally deteriorated image quality on the wafer.
Accordingly, it is an exemplary object of the present invention to provide an exposure apparatus and method that may reduce or eliminate a distortion in the intensity distribution of illumination light when a beam splitting member is provided on an illumination optical path.
All exposure apparatus of one aspect of the present invention includes a condensing optical system for condensing light from a light source to a specified plane, an imaging optical system for imaging the light in the specified plane onto a reticle or a mask or near the reticle or mask, a projection optical system for projecting a pattern on the reticle or the mask onto an object to be exposed, and a beam splitting member that is provided between the condensing optical system and the specified plane, and generates a split beam, wherein 18.3xc2x0xe2x89xa6xcex8xe2x89xa636.4xc2x0, 1.0xe2x89xa6|B|xe2x89xa62.5 and 0.16xe2x89xa6NA2xe2x89xa60.23 are met where xcex8 is an angle formed between a splitting plane in the beam splitting member and a plane perpendicular to an optical axis, B is the magnification of the imaging optical system, and NA2 is a maximum numerical aperture at a light exit side of the imaging optical system. According to this exposure apparatus, the upper and lower lines of the maximum aperture ray among the beams entering the beam splitting member have a small difference in angle of incidence upon the beam splitting member. Thus, it becomes possible to reduce and eliminate a distortion in the intensity distribution of transmitting beams that have transmitted the beam splitting member. Preferably, in this exposure apparatus, the maximum numerical aperture NA3 at a light exit side of the projection optical system satisfies 0.74xe2x89xa6NA3xe2x89xa60.9.
The exposure apparatus may further include a detector for detecting a light amount of the above split beam, and a controller for controlling a light amount of the light source based on the detection result of the detector. Such an exposure apparatus may detect the light amount of light for illuminating a target area, and provides feedback control over the light amount of the light source so that the target area may have illuminance within a specified range. The beam splitting member may have an uncoated reflection surface for reflecting an incident beam so as to generate the split beam. Such a beam splitting member would secure reflectance necessary for the reflection plane without particularly increasing a manufacturing cost.
Further, an exposure method as another aspect of the present invention includes the steps of adjusting, within the angle xcex8, an angle between a beam splitting plane in the beam splitting member in the above exposure apparatus and a plane perpendicular to the optical axis, and illuminating the reticle or the mask using the above exposure apparatus. In addition, the exposure method may further include the steps of detecting a light amount of the beam split by the beam splitting member, and controlling a light amount of the light source based on the result detected by the detecting step. Such an exposure method is an exposure method using the above exposure apparatus, thus performing the operations similar to the above exposure apparatus.
A device fabricating method as still another aspect of the present invention includes the steps of exposing the object by using the above-described exposure apparatus, and performing a specified process for the exposed object. Claims for the device fabricating method that exhibits operations similar to those of the above exposure apparatus cover devices as their intermediate products and finished products. Moreover, such devices include, e.g., semiconductor chips such as LSIs and VLSIs, CCDs, LCDs, magnetic sensors, thin-film magnetic heads, etc.